WO2018172054A1

WO2018172054A1 - Method and clocked converter for operating the input power of light sources with a rapid response

Info

Publication number: WO2018172054A1
Application number: PCT/EP2018/055405
Authority: WO
Inventors: Yanshun Xue; Markus Heckmann
Original assignee: Osram Gmbh
Priority date: 2017-03-23
Filing date: 2018-03-06
Publication date: 2018-09-27
Also published as: DE102017204907A1

Abstract

The invention relates to a method for operating the input power of light sources with a rapid response using a clocked converter which has an output current for operating the light sources. The converter is operated with a low output current such that the current ripple of the output current is high. The invention likewise relates to a clocked converter for operating the input power of light sources with a rapid response, having an input for inputting an input voltage, an output for outputting an output voltage, a switch regulator with a switch, an inductance, and a current valve, wherein the circuit assembly is designed to vary the switch frequency under specific operating conditions in order to increase the current ripple of an output current of the switch regulator and achieve a uniform light emission of the light sources.

Description

METHOD AND ACTUATED CONVERTER FOR OPERATING AN INPUT POWER FAST FOLLOWING LIGHT SOURCES

DESCRIPTION

TECHNICAL FIELD The invention relates to a method and a clocked converter for operating an input power quickly following light sources.

background

The invention is based on a method and a clocked converter for operating an input power quickly following light sources according to the preamble of the main claim.

Fig. 1 shows a known buck converter with the main components also known. A switch SO is connected in series with a freewheeling diode DF. The connection point of the cathode of the freewheeling diode DF and the switch SO is connected to a throttle L. The other terminal of the reactor L is connected to a filter capacitor CF. The other end of the filter capacitor CF and the anode of the diode DF are connected to ground.

The other terminal of the switch SO is together with the ground of the input of the buck converter. The output of the buck converter is parallel to the filter capacitor CF.

Such buck converters are widely used and work satisfactorily. Normally, these circuits are designed so that they have the lowest possible voltage or current ripple, in order to ensure the highest possible power supply to the connected load. This is especially true when one or more of an input power quickly following light sources such as LEDs are connected to such a buck converter. In such light sources, any current ripple is converted directly into a modulation of the light and can adversely affect the quality of light. Just- if a high current ripple leads to an increased electrical load on the light source.

When operating LED, the efficiency must also be considered, which among other things is also a function of the operating current. With extremely small currents, a real LED does not generate any light; with increasing current, the efficiency increases very steeply up to a maximum efficiency. With current flows above the efficiency maximum, the efficiency drops gently. The efficiency maximum is generally at fairly low currents. With commercially available LEDs in sapphire technology in the year of patent filing, it can be assumed that the efficiency maximum lies below 10% of the rated current. When operating above the efficiency maximum, a large current ripple causes a reduction in the average efficiency.

Fig. 2 shows some relevant signals of the known buck converter. The current ISO is the current through the switch SO. It is good to see that the converter works in operation at the gap limit, also referred to as "transition mode." When the switch is switched on, the current increases greatly due to the magnetization of the choke until it is switched off at a certain maximum current In this case, the transistor flows again as soon as the current through the converter diode DF has decayed to the value 0 A. Thus, the converter operates during operation For operating voltages above 200V and power below 1 kW, this mode of operation in the year of the patent application is a usual compromise between component costs, good efficiency and good power density.When LEDs connected to such a step-down converter are now dimmed, a problem arises: For strong dimming positions in the order of a few percent of nominal power and below the current to be emitted to the light sources or LEDs is so small that the special efficiency characteristic of the LEDs comes into play. In addition to the peculiarity of having a maximum, the efficiency at low current spreads very strongly. That

if several LEDs are operated with the same, but very low current, the resulting light emission can be very different, even individual LEDs do not light up while most others are still clearly lit. Not only is the amount of emitted light difficult to predict at low currents, the color locus is also a function of the operating current and difficult to predict at low current.

At low dimming levels, there is another problem, namely that the human eye has a logarithmic sensitivity curve and is better able to distinguish brightness differences at low illuminances than at higher illuminances.

All this means that LEDs connected in series are perceived differently bright at low dimming levels, which is undesirable in many applications.

task

It is an object of the invention to provide a method and a clocked converter, in which at very low operating currents, the light sources have a more uniform light output.

Presentation of the invention

The object is achieved with respect to the method according to the invention with a method for operating an input power quickly following light sources with a clocked converter having an output current for operating the light sources, wherein at low output current of the converter is operated so that the current ripple of Output current is large. This can advantageously be achieved a significant homogenization of the light output of all light sources.

By current ripple is meant here the temporal variation of the output current from a minimum value to a maximum value. By low output current is meant a current less than 10% of the nominal current of the light sources. With large current ripple is a current ripple with a modulation depth of more than 40% meant. The modulation depth MD is calculated as follows: MD = 100% ^* (I-max-lmin) / (lmax + lmin), where Imax is the maximum value of the output current and Imin is the minimum value of the output current.

In a particularly preferred embodiment, the modulation depth of the Stromrippeis is greater than 40%. With such a large modulation depth, the equalization of the light output of the individual light sources is advantageously particularly large.

In another preferred embodiment, at high output current of the clocked converter near the nominal current of the light sources, the transducers are operated so that the current ripple of the output current is small. This advantageously ensures that a particularly high quality of light is achieved when operating the light sources close to the nominal current because the light modulation is minimized.

By high output current is meant in the following an output current of more than 80% of the nominal current of the light sources. By small current ripple is meant a current ripple with a modulation depth of less than 40%. More preferably, the modulation depth of the current ripple when operating near the specified nominal current of the light sources is less than 40%. This advantageously ensures a particularly high quality of light of the emitted light.

In a preferred embodiment, the current ripple is set in the non-latching operation of the clocked converter via the switching frequency. This measure would advantageously allow a simple and accurate adjustment of the desired Stromrippeis with minimal effort.

In another embodiment, the current ripple is set in the latching operation of the clocked converter via the switching frequency and the duty cycle of the switching transistor. This measure makes it possible for transducers which operate in the quiescent mode advantageously to achieve a simple and accurate setting of the desired current fins with minimal additional outlay.

In a further embodiment, with a small output current, the current ripple of the output current is so large that the minimum of the output current becomes zero or becomes negative. This measure advantageously ensures particularly uniform light output of the connected light sources. The object is achieved with respect to the device according to the invention with a clocked converter for operating an input power quickly following light sources, comprising an input for inputting an input voltage, an output for outputting an output voltage, a switching regulator with a switch, an inductance and a Current valve, wherein the circuit arrangement is adapted to vary the switching frequency in certain operating conditions to increase a current ripple of an output current of the switching regulator and to achieve a more uniform light emission of the light source e) len. By increasing the Stromrippeis advantageously has a more uniform light output of the light sources result, since by the ripple voltage and thus current peaks occur which have a better light output of the connected light sources result.

Particularly preferably, the current ripple of the light source current is set by the variation of the switching frequency of the switch. About the switching frequency, the current ripple can be adjusted particularly easily, since the switch-on and the switch-off of the switch can be varied, and thus the current ripple can advantageously be set directly.

In another preferred embodiment, the switch is operated with a pulse width modulation and a current ripple of the light source current is set by the variation of the duty cycle of the pulse width modulation and by the variation of the switching frequency. This embodiment is used in transducers in lopsided operation, since the average current can be adjusted very easily via the current gap, and via the switch-on and off

25 switching times of the switch of the current ripple can be set independently of the average current.

In this case, the converter is particularly preferably a buck converter. This is the optimal transducer topology for operating some light sources such as e.g. Light emitting diodes on a supply network, since the forward voltage of the series-connected

30 sources is less than the supply voltage. If, for certain applications, a converter with insulation between the primary side and the secondary side is necessary, the converter is preferably a flyback converter, which advantageously ensures this isolation.

In a particularly preferred embodiment of the buck converter, the flow control valve is a lower switching transistor. This measure advantageously ensures a particularly efficient and energy-saving operation of the converter, namely the forced continuous conduction mode (FCCM), also referred to as forced non-latching operation Thus, over a wide output voltage and output current range are switched voltage-free, which advantageously greatly increases the efficiency.

Further advantageous developments and refinements of the clocked converter according to the invention and of the method for operating the clocked converter emerge from further dependent claims and from the following description.

Brief description of the drawings

Further advantages, features and details of the invention will become apparent from the following description of exemplary embodiments and with reference to the drawings, in which the same or functionally identical elements are provided with identical reference numerals. Showing:

Fig. 1 shows a schematic circuit diagram of a known buck converter according to the prior art

Fig. 2 is a timing diagram of the known buck converter

3 shows some relevant signals of the operation of the buck converter in non-gap operation at 50 kHz switching frequency, 4 shows some relevant signals of the operation of the buck converter in the non-gap operation at 200kHz switching frequency,

Fig. 5 some relevant signals of the operation of the buck converter in lückenenden

Operation at 50kHz switching frequency,

Fig. 6 some relevant signals of the operation of the buck converter in lückenden

Operation at 200kHz switching frequency,

7 is a schematic circuit diagram of a known flyback converter,

8 shows some relevant signals of the operation of the flyback converter in non-gap operation at 50 kHz switching frequency,

9 shows some relevant signals of the operation of the flyback converter in non-gap operation at a 200 kHz switching frequency,

Fig. 10 shows some relevant signals of the operation of the flyback converter in the latching

Operation at 50kHz switching frequency,

Fig. 1 1 some relevant signals of the operation of the flyback converter in lückenden

Operation at 200kHz switching frequency,

Fig. 12 is a schematic circuit diagram of a known deep-setting half-bridge

Preferred embodiment of the invention

The basic idea of the invention is to operate a converter in such a way that it does not generate the lowest possible current ripple on the LEDs at low output currents, but rather specifically generates an increased current ripple on the LEDs. The high current ripple at low currents produces a similar effect as if the LEDs were dimmed by means of pulse width modulation. The

LEDs are operated for a short time with a current that is closer to the nominal current of these LEDs than in the "pause times" between the current pulses.Thus, the series differences of the individual LEDs with respect to the light emission at low current not so much weight and the light output of the LEDs will be At a low DC current, the series spread of the LEDs is much more significant and the output of each LED is more uneven, and the converter is now operated to control the output ripple to maximize the effect that the ripple current produces at low currents use. For this purpose, the converter is no longer operated at the gap limit, but either in non-leaking operation or in lopsided operation. At lower switching frequencies, the maximum inductor current at which the upper switching transistor SO is turned off is larger and the minimum inductor current at which the upper transistor SO is turned on again is smaller. Thus, the power output of the converter can be kept equal, although the frequency decreases and the on and off threshold is changed. The situation is similar in the case of lopsided operation, the longer the current gaps, the higher and longer the current pulses must be, so that the average current remains unchanged. By reducing the frequency and adjusting the duty cycle, the current ripple can be adjusted and, at the same time, the mean value of the LED current can be kept unchanged. This is used in the present invention to artificially increase the ripple in the stream at low current levels to achieve equalization of the light output of the individual LEDs in the strand. With a higher current ripple, as described above, a similar effect arises as with operation of the LEDs with pulse width modulation. The LEDs then have a higher light modulation at low currents, but shine more evenly, which is the better compromise overall, especially because the light modulation takes place in the kHz range and is not visible to humans. Preferably, at low dimming positions, ie at low currents, a larger current ripple is generated for the operation of the LEDs and at high dimming positions, ie at high current and high light output, the smallest possible ripple is generated in order to achieve a high quality of light and efficiency at high illuminance levels. FIG. 3 shows the current profile 31 through the LEDs as well as the gate voltage and the

Drain voltage of the switching transistor SO of the buck converter. It is good to see that the current through the LEDs is not uniform, but has a relatively large ripple of about 5mApp, which causes the light output of the LEDs to become more uniform. The average current through the LED strand 5 is approximately 10mA. The switching frequency here is about 50kHz. The voltage in the maximum current accordingly increases a little, but less than one would expect. The variation of the forward voltage is very low, but nonetheless leads to a visibly different light output at low currents. For the sake of completeness, the gate voltage 33 and the drain-source voltage 35 of the switching transistor SO are shown here. It is good to see that the duty cycle is not 50%, but the ON time of the switching transistor SO is shorter than the OFF time.

Fig. 4 shows the same circuit with the same duty cycle, but a higher switching frequency of the switching transistor SO. Here, the switching frequency is about 200kHz. The average current through the LED strand 5 is here also 10mA, the current ripple, however, only 0.3mApp. This current is close to DC, which is good and desirable at high currents.

The current ripple on the LEDs can therefore be adjusted by the frequency in the non-lapping operation.

Fig. 5 shows the situation of Fig. 3 in the lopsided operation at also 50kHz switching frequency of the converter transistor SO. The current 51, which corresponds to the average LED current IL is here as in Figures 3 and 4 also 10mA. The current ripple is here at about 15mApp, which corresponds to one and a half times the average LED current IL. The duty cycle is around 10%.

Fig. 6 shows the situation also in the lopsided operation at 200kHz. The current 61, which corresponds to the average LED current IL, is here again 10mA. The current ripple, however, is only about 3mApp. The duty cycle is about 15% higher than at 50kHz.

In lückenden operation of the current ripple can therefore be varied by the variation of the frequency and the duty cycle at a given output current. Preferably, the converter is now operated so that it operates at high output currents near the nominal current of the LEDs to be operated with the smallest possible Ausgangsripplestrom to ensure high quality light with low modulation. However, the lower the output current, the more so the LEDs are dimmed, the greater the ripple current in the output current IL is set to ensure a uniform light output of the LEDs. In particular, the ripple current is increased at low output currents less than 10% of the nominal current to achieve this effect. This method of operation is in no way limited to buck converters, as shown in the following figures. Any converter that has the appropriate structure for non-latching operation may interfere with the current ripple and thus perform the method.

Fig. 7 shows a schematic circuit diagram of a known flyback converter. Here, the converter also consists of a converter transistor SO, a converter diode D and a filter capacitor CF. As a flyback converter, it has a converter transformer comprising a primary coil L1 and a secondary coil L2. As a load, an LED strand 5 is also connected.

The current ripple of the LED current IL can also be set with this converter topology, as shown in the following figures.

FIGS. 8 and 9 show the variation of the current ripple in the flyback converter in non-latching operation.

The output current 81 of the converter, which in turn corresponds to the LED current IL, is again set to an average value of 10 mA in FIG. 8. The current ripple of the output current 81 is here at 9 mApp at 50 kHz switching frequency of the converter transistor. It can easily be seen at the gate voltage 85 and at the drain voltage 83 of the converter transistor SO that the duty ratio is approximately 50%.

Fig. 9 shows the situation at 200kHz switching frequency. Again, the average current IL is 10mA, as can be clearly seen in the output current 91. However, the current ripple at this frequency is only 1.5mApp. It can clearly be seen from the gate voltage 95 and from the drain-source voltage 93 that the duty cycle likewise lies again at approximately 50%. FIGS. 10 and 11 show the situation in the latching operation of the flyback converter.

Fig. 10 shows the output current 101 at 50kHz and a duty cycle of about 10%. The current ripple of the current 101 here is about 18mApp. That is 1, 8 times the output current. At the gate voltage 105 and the drain source voltage 103, the duty cycle can be seen.

Fig. 1 1 again shows the operation at 200kHz. The output current 1 1 1 here again has a mean value of 10mA, and has a current ripple of only 1, 5mApp. The drain-source voltage 1 13 and the gate source voltage 1 15 show a duty cycle of about 15%.

Fig. 12 shows a deep-setting half-bridge which also enables the above-mentioned modes of operation. The essential difference to the topology explained in FIG. 1 is the replacement of the converter diode DF by a lower transistor SU. This results in a half-bridge structure, wherein the half-bridge is connected in parallel to the input of the converter. The positive input is at a DC potential of about 400V, the negative input is a reference potential. The converter inductor L is connected to the half-bridge center HSS, the other terminal of the converter inductor L together with the reference potential forms the output LED + / LED- of the converter. A filter capacitor CF is connected in parallel with the LED + / LED output of the converter. In addition, operation in Forced Continuous Conduction Mode (FCCM for short) is also possible with the deepening half bridge. Operation in the FCCM allows avoiding switching losses and also allows control of the current ripple by varying the operating frequency.

Of course, the method is not limited to the transducer topologies described above. The method can be carried out with almost every clocked converter, in particular further suitable converter topologies are mentioned below: A half-bridge zeta converter, as described for example in WO 2015 044 846 A2.

A so-called "Flybuck" converter, which is a combination of a buck converter and a flyback converter, which also supplies additional isolated output voltages to the transformer during the demagnetization phase Flybuck converter is described here: Texas Instruments Application Report AN-2292: "Designing an Isolated (Flybuck) Converter".

An LLC converter, which is a resonant converter with an LLC resonant circuit. This transducer topology is well known in the art and is described, for example, here: Texas Instruments Application Note AN-2644: "An introduction to LLC resonant half-bridge converter".

BEZUGSZEICHENLISTSE

1 circuit arrangement

5 LEDs

31, 41, 51, 61, 81, 91, 101, 111 output current IL

33, 43, 53, 63, 83, 93, 103, 113 drain-source voltage of the switching transistor

SO

35, 45, 55, 65, 85, 95, 105, 115 gate-source voltage of the switching transistor

SO

UE input voltage

UA output voltage

UM voltage at the input side of the reactor IL inductor current

Claims

claims

1 . Method for operating an input power quickly following light sources (5) with a clocked converter having an output current for driving the light sources, having the following properties:

At low output current, the converter is operated so that the current ripple of the output current is large.

2. The method according to claim 1, characterized in that the modulation depth of the Stromrippeis is greater than 40%.

3. - Method according to claim 1, characterized in that at high output current of the clocked converter near the nominal current of the light sources (5), the converter is operated so that the current ripple of the output current is small.

4. The method according to claim 3, characterized in that the modulation depth of the Stromrippeis is less than 40%.

5. The method according to any one of the preceding claims, characterized in that the current ripple is set in the non-latching operation of the clocked converter via the switching frequency.

6. The method according to any one of the preceding claims, characterized in that the current ripple is set in the latching operation of the clocked converter via the switching frequency and the duty cycle of the switching transistor.

7. The method according to any one of the preceding claims, characterized in that at low output current of the current ripple of the output current is so large that the minimum of the output current is zero or negative.

8. A clocked converter for operating an input power quickly following light sources, comprising:

an input for inputting an input voltage (UE),

an output for outputting an output voltage (UA), a switching regulator with a switch (SO), an inductance (L) and a flow control valve (D),

wherein the circuit arrangement is set up to vary the switching frequency in certain operating states in order to increase a current ripple of an output current (IL) of the switching regulator and to achieve a more uniform light emission of the light sources (5).

9. A clocked converter according to claim 8, characterized in that a current ripple of the light source current is adjusted by the variation of the switching frequency of the switch.

A clocked converter according to claim 8, characterized in that it is arranged to operate the switch with a pulse width modulation and to set a current ripple of the light source current by the variation of the duty cycle of the pulse width modulation and by the variation of the switching frequency.

1 1. Clocked converter according to one of the preceding claims 8 to 10, characterized in that it is a buck converter.

12. A clocked converter according to one of the preceding claims 8 to 10, characterized in that it is a flyback converter.

Clocked converter according to one of the preceding claims 8 to 10, characterized in that it is a boost converter, or is a LLC converter, or is a Cuk converter, or is a sepia converter, or is a half-bridge zeta converter, or a FlyBuck converter, or is a Z converter.

Clocked converter according to one of the preceding claims 8 to 13, characterized in that the flow control valve is a switching transistor (SU).